As of 2013, there are estimated to be about 100 “uncontacted” tribes left in the world, mostly in thick forested areas like New Guinea and the Amazon rainforest (Uncontacted peoples). But most of these would be “minimally contacted” tribes. Some of these tribes have at least seen airplanes and helicopters that have photographed them. Some have learned about the rest of the world from encounters with neighboring tribes that have been contacted, and some have picked up a few Western goods through trade or gifting. And some are tribes that have already fled their land and withdrawn deeper into the wilderness—these do know about “the rest of the world,” and they have good reason to fear it and avoid it.

But if you're talking about tribes that have had no contact with the outside world, it is probably much lower than 100. But this is harder to measure. It would be very hard for us to find out about a tribe’s existence without the tribe finding out about ours.

Government policy in most of the countries that still have uncontacted tribes is generally to try and leave them alone, although missionaries and tourists sometimes violate the policy. This is in part because uncontacted tribes may have no immunity to “civilized world” diseases—when the Nukak people of Colombia were contacted in 1981, about half the tribe died of respiratory disease. Narco-traffickers and land grabbers moved in, and the surviving Nukak were forced to move to a town where they’re stuck in wretched conditions. Other tribes have been wiped out or severely reduced by having their land seized for mining, agriculture, timber, etc., or by slaving, or by military or guerrilla actions. This article tells some of the story: The Lost Tribes of the Amazon.

More recently, in 2014, three members of a previously uncontacted tribe in Brazil encountered a team from Brazil’s National Indian Foundation (FUNAI), and unfortunately caught influenza. They were treated, but it’s feared that they may have brought it back to their tribe. Making matters worse, the tribe in question may have been fleeing attacks by cocaine traffickers. See: Members of previously uncontacted tribe infected with flu. Here are the three:

Probably the best known uncontacted tribes are the people of North Sentinel Island in the Indian Ocean. Even they have had some contact with the outside world—they know we’re out here, but most attempts at contact have been met with arrows and spears. Between 1967 and 1996, there were a few attempts to make peaceful contact with them by leaving them gifts, so the Sentinelese have at least accepted a few Western goods, such as aluminum cookware, and we have a few close-up photos of them. Several people attempting to make contact were hurt or killed, including the director of a National Geographic documentary, who was speared in the leg during a 1974 contact attempt that ended badly. Since 1996 the Indian government policy has been to leave them alone. In 2006, the Sentinelese killed two fishermen who were in their waters illegally—and then drove away the Indian Navy helicopter sent to retrieve them, with a volley of arrows.

They’re not “ignorant of modern technological advances”. But they evidently want nothing to do with them.